Methods of selectively separating CO2 from a multicomponent gaseous stream

ABSTRACT

Methods are provided for the selective removal of CO 2  from a multicomponent gaseous stream to provide a CO 2  depleted gaseous stream having at least a reduction in the concentration of CO 2  relative to the untreated multicomponent gaseous stream. In the subject methods, the multicomponent gaseous stream is contacted with CO 2  nucleated water under conditions of selective CO 2  clathrate formation, where the CO 2  nucleated water serves as a liquid absorbent or adsorbent to produce a CO 2  clathrate slurry and CO 2  depleted gaseous stream. In a preferred embodiment, the CO 2  clathrate slurry is then decomposed to produce CO 2  gas and nucleated water. The subject methods find use in a variety of applications where it is desired to selectively remove CO 2  from a multicomponent gaseous stream, including chemical feedstock processing applications and air emissions control applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No.09/067,937 filed on Apr. 28, 1998, which application is a continuationin part of application Ser. No. 08/923,172, filed on Sep. 4, 1997 nowabandoned, which application is a continuation of application Ser. No.08/643,151 filed on Apr. 30, 1996, now U.S. Pat. No. 5,700,311, thedisclosures of which are herein incorporated by reference.

INTRODUCTION

1. Field of the Invention

The field of this invention is selective absorption of CO₂ gas.

2. Introduction

In many applications where mixtures of two or more gaseous componentsare present it is often desirable to selectively remove one or more ofthe component gases from the gaseous stream. Of increasing interest in avariety of industrial applications, including power generation, chemicalsynthesis, natural gas upgrading, and conversion of methane hydrates tohydrogen and CO₂, is the selective removal of CO₂ from multicomponentgaseous streams.

An example of where selective CO₂ removal from a multicomponent gaseousstream is desirable is the processing of synthesis gas or syngas. Syngasis a mixture of hydrogen, carbon monoxide and CO₂ that is readilyproduced from fossil fuels and finds use both as a fuel and as achemical feedstock. In many applications involving syngas, the carbonmonoxide is converted to hydrogen and additional CO₂ via the water-gasshift process. It is then often desirable to separate the CO₂ from thehydrogen to obtain a pure H₂ stream for subsequent use, e.g. as a fuelor feedstock.

As man made CO₂ is increasingly viewed as a pollutant, another area inwhich it is desirable to separate CO₂ from a multicomponent gaseousstream is in the area of pollution control. Emissions from industrialfacilities, such as manufacturing and power generation facilities, oftencomprise CO₂. In such instances, it is often desirable to at leastreduce the CO₂ concentration of the emissions. The CO₂ may be removedprior to combustion in some cases and post combustion in others.

A variety of processes have been developed for removing or isolating aparticular gaseous component from a multicomponent gaseous stream. Theseprocesses include cryogenic fractionation, selective adsorption by solidadsorbents, gas absorption, and the like. In gas absorption processes,solute gases are separated from gaseous mixtures by transport into aliquid solvent. In such processes, the liquid solvent ideally offersspecific or selective solubility for the solute gas or gases to beseparated.

Gas absorption finds widespread use in the separation of CO₂ frommulticomponent gaseous streams. In CO₂ gas absorption processes thatcurrently find use, the following steps are employed: (1) absorption ofCO₂ from the gaseous stream by a host solvent, e.g. monoethanolamine;(2) removal of CO₂ from the host solvent, e.g. by steam stripping; and(3) compression of the stripped CO₂ for disposal, e.g. by sequestrationthrough deposition in the deep ocean or ground aquifers.

Although these processes have proved successful for the selectiveremoval of CO₂ from a multicomponent gaseous stream, they are energyintensive. For example, using the above processes employingmonoethanolamine as the selective absorbent solvent to remove CO₂ fromeffluent flue gas generated by a power plant often requires 25 to 30% ofthe available energy generated by the plant. In most situations, thisenergy requirement, as well as the additional cost for removing the CO₂from the flue gas, is prohibitive.

Accordingly, there is continued interest in the development of lessenergy intensive processes for the selective removal of CO₂ frommulticomponent gaseous streams. Ideally, alternative CO₂ removalprocesses should be simple, require inexpensive materials and low energyinputs, and be low in cost for separation and sequestration of the CO₂.For applications in which it is desired to effectively sequester theseparated CO₂, of particular interest would be the development ofalternative CO₂ absorbents or adsorbents from which the absorbed oradsorbed CO₂ could be effectively and efficiently stripped at highpressure prior to further compression and sequestration. Of particularinterest would be the development of a system which minimizes parasiticenergy losses for all process steps necessary to produce a high pressureCO₂ gas stream for disposal (sequestration and utilization).

Relevant Literature

Patents disclosing methods of selectively removing one or morecomponents from a multicomponent gaseous stream include: U.S. Pat. Nos.3,150,942; 3,838,553; 3,359,744; 3,479,298; 4,253,607; 4,861,351;5,387,553; 5,434,330; 5,562,891 and 5,600,044.

Reports summarizing currently available processes for reducing the CO₂content of multicomponent gaseous streams, such as coal fired powerplant emissions, include: Smelser, S. C. et al., "Engineering andEconomic Evaluation of CO₂ Removal From Fossil-Fuel-Fired Powerplants,Vol. 1: Pulverized-Coal-Fired Powerplants," EPRI IE-7365 Vol. 1 and Vol.2; Coal Gasification-Combined Cycle Power Plants, EPRI IE-7365, Vol. 2.

Other publications discussing CO₂ clathrate formation include Japaneseunexamined patent application 3-164419, Nishikawa et al., "CO₂ ClathrateFormation and its Properties in the Simulated Deep Ocean," EnergyConvers. Mgmt. (1992) 33:651-657; Saji et al., "Fixation of CarbonDioxide by Clathrate-Hyrdrate," Energy Convers. Mgmt. (1992) 33:643-649; Austvik & L.o slashed.ken, "Deposition of CO₂ on the Seabed inthe Form of Clathrates," Energy Convers. Mgmt. (1992) 33: 659-666;Golumb et al., "The Fate of CO₂ Sequestered in the Deep Ocean," EnergyConvers. Mgmt. (1992) 33: 675-683; Spencer, "A Preliminary Assessment ofCarbon Dioxide Mitigation Options," Annu. Rev. Energy Environ. (1991)16: 259-273; Spener & North, "Ocean Systems for Managing the GlobalCarbon Cycle," Energy Convers. Mgmt. (1997) 38 Suppl.: 265-272; andSpencer & White, "Sequestration Processes for Treating MulticomponentGas Streams," Proceedings of 23^(rd) Coal and Fuel Systems Conference,Clearwater, Fla. (March 1998).

SUMMARY OF THE INVENTION

Methods are provided for the selective removal of CO₂ from amulticomponent gaseous stream. In the subject methods, a multicomponentgaseous stream comprising CO₂ is contacted with CO₂ nucleated water inwhich hydrate precursors are contained under conditions of selective CO₂clathrate formation, conveniently in a reactor. The CO₂ nucleated water(hydrate precursor water) employed in the subject invention comprisesdissolved CO₂ in the form of CO₂ hydrate or clathrate precursors, wherethe precursors are in metastable form. The CO₂ nucleated water (hydrateprecursor water) may either be formed in situ in the reactor or in aseparate reactor, where the water may be fresh or salt water. Once theCO₂ nucleated water is formed, it serves as a selective CO₂ liquidabsorbent or adsorbent. Upon contact of the gaseous stream with the CO₂nucleated water, CO₂ is selectively absorbed or adsorbed from thegaseous stream by the CO₂ nucleated water and concomitantly fixed as CO₂clathrates to produce a CO₂ depleted multicomponent gaseous stream and aslurry of CO₂ clathrates. The resultant CO₂ depleted multicomponentgaseous stream is then separated from the CO₂ clathrate slurry, eitherin the reactor itself or in a downstream separator. In a preferredembodiment, the resultant slurry is then treated in a manner sufficientto decompose the CO₂ hydrates to produce a moderate to high pressure CO₂gas and CO₂ nucleated water. The process is suitable for use with a widevariety of multicomponent gaseous streams.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides a flow diagram of a first preferred embodiment of thesubject invention.

FIG. 2 provides a flow diagram of a second preferred embodiment of thesubject invention.

DETAILED DESCRIPTION OF THE INVENTION

Methods for selectively removing CO₂ from a multicomponent gaseousstream are provided. In the subject methods, a multicomponent gaseousstream is contacted with CO₂ nucleated water under conditions ofselective CO₂ clathrate formation, conveniently in a reactor. The CO₂nucleated water may be prepared in situ in the reactor, or in a separatereactor, where the water may be either fresh or salt water. Upon contactof the gaseous stream with the CO₂ nucleated water, CO₂ is selectivelyabsorbed from the gaseous stream by the CO₂ nucleated water andconcomitantly fixed in the form of the CO₂ clathrates. Contact resultsin the production of a CO₂ depleted gaseous stream and a slurry of CO₂clathrates, which are then separated. In a preferred embodiment, the CO₂clathrate or hydrate slurry is treated to decompose the CO₂ hydrates toproduce a moderate to high pressure CO₂ gas and CO₂ nucleated water. Thesubject invention finds use in the treatment of a variety ofmulticomponent gaseous streams.

Before the subject invention is described further, it is to beunderstood that the invention is not limited to the particularembodiments of the invention described below, as variations of theparticular embodiments may be made and still fall within the scope ofthe appended claims. It is also to be understood that the terminologyemployed is for the purpose of describing particular embodiments, and isnot intended to be limiting. Instead, the scope of the present inventionwill be established by the appended claims.

In this specification and the appended claims, the singular forms "a,""an," and "the" include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood to one of ordinary skill in the art to which this inventionbelongs.

Critical to the subject invention is the use of CO₂ nucleated watercontaining CO₂ hydrate precursors as a liquid that selectively absorbsor adsorbs the gaseous CO₂ from the multicomponent gas phase stream. TheCO₂ nucleated water employed in the subject invention comprisesdissolved CO₂ in the form of CO₂ hydrate or clathrate precursors, wherethe precursors are in metastable form. The mole fraction of CO₂ in theCO₂ nucleated water ranges from about 0.01 to 0.10, usually from about0.02 to 0.08, more usually from about 0.04 to 0.06. The temperature ofthe CO₂ nucleated water will typically range from about -1.5 to 10° C.,preferably from about -1.5 to 5° C., and more preferably from about -1.5to 0.5° C.

CO₂ nucleated water employed in the subject methods as the selectiveliquid absorbent or adsorbent may be prepared using any convenientmeans. One convenient means of obtaining CO₂ nucleated water isdescribed in U.S. application Ser. No. 08/291,593, filed Aug. 16, 1994,now U.S. Pat. No. 5,562,891, the disclosure of which is hereinincorporated by reference. In this method CO₂ is first dissolved inwater using any convenient means, e.g. bubbling a stream of CO₂ gasthrough the water, injection of CO₂ into the water under conditions ofsufficient mixing or agitation to provide for homogeneous dispersion ofthe CO₂ throughout the water, and the like, where the CO₂ source that iscombined with the water in this first stage may be either in liquid orgaseous phase. Where gaseous CO₂ is combined with water to make the CO₂nucleated water, the gaseous CO₂ will typically be pressurized, usuallyto partial pressures ranging between 6 to 50 atm, more usually betweenabout 20 to 40 atm. The CO₂ may be derived from any convenient source.In a preferred embodiment, at least a portion of the CO₂ is gaseous CO₂obtained from a CO₂ hydrate slurry decomposition step, as described ingreater detail below. The water in which the CO₂ is dissolved may befresh water or salt water, e.g. sea water. An example of a process whichemploys freshwater based CO₂ nucleated water is provided in FIG. 1 whilean example of a process which employs sea water based CO₂ nucleatedwater is provided in FIG. 2. The temperature of the water will generallyrange from about -1.5 to 10° C., usually from about -1.5 to 5°C., moreusually from about 0 to 1° C.

The water that is used to produce the nucleated water may be obtainedfrom any convenient source, where convenient sources include the deepocean, deep fresh water aquifers, powerplant cooling ponds, and thelike, and cooled to the required reactor conditions. In certainembodiments, the nucleated water may be recycled from a downstreamsource, such a clathrate slurry heat exchanger/decomposition source (asdescribed in greater detail below) where such recycled nucleated watermay be supplemented as necessary with additional water, which water mayor may not be newly synthesized nucleated water as described above.

The amount of CO₂ which is dissolved in the water will be determined inview of the desired CO₂ mole fraction of the CO₂ nucleated water to becontacted with the gaseous stream. One means of obtaining CO₂ nucleatedwater having relatively high mole fractions of CO₂ is to produce aslurry of CO₂ clathrates and then decompose the clathrates by loweringthe pressure and/or raising the temperature of the slurry to release CO₂and regenerate a partially nucleated water stream. Generally, nucleatedwater having higher mole fractions of CO₂ are desired because it morereadily accepts CO₂ absorption or adsorption and excludes formation ofother hydrate compounds. By high mole fraction of CO₂ is meant a molefraction of about 0.06 to 0.10, usually from about 0.07 to 0.09.

The production of CO₂ nucleated water may conveniently be carried out ina nucleation reactor. The reactor may be packed with a variety ofmaterials, where particular materials of interest are those whichpromote the formation of CO₂ nucleated water with hydrate precursors andinclude: stainless steel rings, carbon steel rings, and the like, topromote gas-liquid contact. To ensure that the optimal temperature ismaintained in the nucleation reactor, active coolant means may beemployed. Any convenient coolant means may be used, where the coolantmeans will typically comprise a coolant medium housed in a containerwhich contacts the reactor, preferably with a large surface area ofcontact, such as coils around and/or within the reactor or at least aportion thereof, such as the lower portion of the reactor. Coolantmaterials or media of interest include ammonia, HCFCs, and the like,where a particular coolant material of interest is ammonia, where theammonia is maintained at a temperature of from about 0 to 10° C. Thesurface of the cooling coils, or a portion thereof, may be coated with acatalyst material, such as an oxide of aluminum, iron, chromium,titanium, and the like, to accelerate CO₂ hydrate precursor formation.Additionally, hydrate crystal seeding or a small (1-3 atm) pressureswing may be utilized to enhance hydrate precursor formation.

In a preferred embodiment of the subject invention, the CO₂ nucleatedwater is prepared by contacting water (e.g. fresh or salt water) withhigh pressure, substantially pure CO₂ gas provided from an external highpressure CO₂ gas source. In this embodiment, the water is contacted withsubstantially pure CO₂ gas which is at a pressure that is about equal toor slightly above the total multicomponent gaseous stream pressure. Assuch, the pressure of the substantially pure CO₂ gas typically ranges inmany embodiments from about 5 to 7 atm above the multicomponent gaseousstream pressure, and may be 10 to 50, usually 10 to 40 and more usually15 to 30 atm above the CO₂ partial pressure of the multicomponentgaseous stream (CO₂ overpressure stimulation of hydrate precursor andhydrate formation). By substantially pure is meant that the CO₂ gas isat least 95% pure, usually at least 99% pure and more usually at least99.9% pure. Advantages realized in this preferred embodiment include theproduction of CO₂ saturated water that comprises high amounts ofdissolved CO₂, e.g. amounts (mole fractions) ranging from about 0.02 to0.10, usually from about 0.04 to 0.08. Additional advantages include theuse of relatively smaller nucleation reactors (as compared to nucleationreactors employed in other embodiments of the subject invention) and theproduction of more CO₂ selective nucleated water. In those embodimentswhere small nucleation reactors are employed, it may be desirable tobatch produce the CO₂ saturated water, e.g. by producing the totalrequisite amount of CO₂ saturated water in portions and storing thesaturated water in a high pressure reservoir. The CO₂ saturated water isreadily converted to nucleated water, i.e. water laden with CO₂ hydrateprecursors, using any convenient means, e.g. by temperature cycling,contact with catalysts, pressure cycling, etc.

The first step of the subject method is to contact the multicomponentgaseous stream with CO, nucleated water under conditions of CO₂clathrate formation, preferably under conditions of selective CO₂clathrate formation. The CO₂ nucleated water may be contacted with thegaseous stream using any convenient means. Preferred means of contactingthe CO₂ nucleated water with the gaseous stream are those means thatprovide for efficient absorption or adsorption of the CO₂ from the gasthrough solvation of the gaseous CO₂ in the liquid phase CO₂ nucleatedwater. Means that may be employed include concurrent contacting means,i.e. contact between unidirectionally flowing gaseous and liquid phasestreams, countercurrent means, i.e. contact between oppositely flowinggaseous and liquid phase streams, and the like. Thus, contact may beaccomplished through use of spray, tray, or packed column reactors, andthe like, as may be convenient.

Generally, contact between the multicomponent gaseous stream and thenucleated water is carried out in a hydrate or clathrate formationreactor. The reactor may be fabricated from a variety of materials,where particular materials of interest are those which promote theformation of CO₂ clathrates or hydrates and include: stainless steel,carbon steel, and the like. The reactor surface, or a portion thereof,may be coated with a catalyst material, such as an oxide of aluminum,iron, chromium, titanium, and the like, to accelerate CO₂ hydrateformation. To ensure that the optimal temperature is maintained in thehydrate formation reactor, active coolant means may be employed. Anyconvenient coolant means may be used, where the coolant means willtypically comprise a coolant medium housed in a container which contactsthe reactor, preferably with a large surface area of contact, such ascoils around or within the reactor or at least a portion thereof, suchas the exit plenum of the reactor. Coolant materials or media ofinterest include ammonia, HCFCs and the like, where a particular coolantmaterial of interest is ammonia, where the ammonia is maintained at atemperature of from about 0 to 10° C. Where the reactor comprises gasinjectors as the means for achieving contact to produce clathrates, thereactor may comprise 1 or a plurality of such injectors. In suchreactors, the number of injectors will range from 1 to about 20 or more,where multiple injectors provide for greater throughput and thus greaterclathrate production. Specific examples of various reactors that may beemployed for clathrate production are provided in U.S. application Ser.No. 09/067,937, the disclosure of which is herein incorporated byreference.

The clathrate formation conditions under which the gaseous and liquidphase streams are contacted, particularly the temperature and pressure,may vary but will preferably be selected so as to provide for theselective formation of CO₂ clathrates, to the exclusion of clathrateformation of other components of the multi-component gaseous stream,unless such gases can be efficiently removed with the CO₂, e.g. H₂ S.Generally, the temperature at which the gaseous and liquid phases arecontacted will range from about -1.5 to 10° C., usually from about -1.5to 5° C., more usually from about 0 to 1° C. The CO₂ partial pressure orthe total pressure in the reactor will generally be at least about 3-5atm to 6 atm, usually at least about 8 atm, and more usually at leastabout 10 atm, but will generally not exceed 60 atm, and more usuallywill not exceed 30 atm, where higher pressures are required when highertemperatures are employed, and vice versa, or where high pressure gasesare conveniently available from the upstream process or processes.

Upon contact of the gaseous stream with the CO₂ nucleated water, CO₂ isselectively absorbed or adsorbed from the gaseous stream into the CO₂nucleated water liquid phase. The absorbed or adsorbed CO₂ isconcomitantly fixed as solid CO₂ clathrates in the liquid phase. Contactbetween the gaseous and liquid phases results in the production of a CO₂depleted multicomponent gaseous stream and a slurry of CO₂ clathrates.In the CO₂ depleted multicomponent gaseous stream, the CO₂ concentrationis reduced by at least about 50%, usually by at least about 70%, andmore usually by at least about 90%, as compared to the untreatedmulticomponent gaseous stream. In other words, contact of themulticomponent gaseous stream with the CO₂ nucleated water results in atleast a decrease in the concentration of the CO₂ of the gaseous phase,where the decrease will be at least about 50%, usually at least about70%, more usually at least about 90%. In some instances theconcentration of CO₂ in the gaseous phase may be reduced to the levelwhere it does not exceed 1% (v/v), such that the treated gaseous streamis effectively free of CO₂ solute gas.

As discussed above, the CO₂ absorbed or adsorbed by the CO₂ nucleatedwater is concomitantly fixed in the form of stable CO₂ clathrates.Fixation of the CO₂ in the form of stable CO₂ clathrates results in theconversion of the CO₂ nucleated water to a slurry of CO₂ clathrates. Theslurry of CO₂ clathrates produced upon contact of the gaseous streamwith the CO₂ nucleated water comprises CO₂ stably fixed in the form ofCO₂ clathrates and water. Typical mole fractions of CO₂ in stableclathrates are 0.12 to 0.15, as compared to 0.02 to 0.04 in the CO₂nucleated water.

As described above, the CO₂ nucleated water that serves as the selectiveliquid absorbent or adsorbent for the CO₂ solute gas of themulticomponent gaseous stream is produced by dissolving CO₂ in water. Assuch, in some embodiments of the subject invention, CO₂ free water maybe contacted with the multicomponent gaseous stream under appropriateconditions to first produce the CO₂ nucleated water, where contact willbe subsequently maintained to produce the CO₂ clathrate slurry. In otherwords, the separate steps of CO₂ nucleated water production and thecontact between the gaseous stream and the CO₂ nucleated water arecombined into one continuous process.

In a preferred embodiment of the subject invention, the CO₂ containinggas and the nucleated water are injected into the reactor in directionsnormal to each other, e.g. the CO₂ containing gas is injected into thehydrate reactor along the centerline of the reactor while the nucleatedwater is injected into the reactor from the sides of the reactor suchthat the multicomponent gaseous stream is injected into the reactor in adirection substantially normal to the CO₂ nucleated water. In certainpreferred embodiments, the water flows into an annulus surrounding themulticomponent gaseous stream containing the CO₂ and the two streams mixin the hydrate reaction zone. Thus, this nucleated water educts the CO₂containing gaseous stream into the hydrate reactor. This embodiment ofthe subject invention provides a number of advantages over otherembodiments of the invention, as described above. These benefits includeone or more of: use of injection nozzels with smaller diameters;formation of CO₂ hydrates at lower equivalent pressures, e.g. down to 3to 6 atm; reduction in the number of CO₂ hydrate reactors needed for agiven volume of gas to be treated; enhancement of the mixing of themulticomponent gaseous stream with the nucleated water; and enhancementin the selectivity of CO₂ absorption or adsorption from themulticomponent gaseous stream.

In certain embodiments, an analogous step is employed in the nucleationreactor to produce the CO₂ nucleated water. In these embodiments, theCO₂ containing gaseous stream and the water are injected into thereactor in directions normal to each other, e.g. the CO₂ containinggaseous stream is injected along the centerline of the nucleationreactor and the water is injected into the nucleation reactor in adirection substantially normal to the centerline of the reactor. Incertain preferred embodiments, the water flows into an annulussurrounding the gaseous stream containing the CO₂ and the two streamsmix in the reaction zone.

The second step of the subject method is the separation of the treatedgaseous phase from the CO₂ clathrate slurry. As convenient, the gaseousphase may be separated from the slurry in the reactor or in a downstreamgas-liquid separator. Any convenient gas-liquid phase separation meansmay be employed, where a number of such means are known in the art. Inmany preferred embodiments, the gas-liquid separator that is employed isa horizontal separator with one or more, usually a plurality of, gas offtakes on the top of the separator. The subject invention provides forextremely high recovery rates of the multicomponent gaseous stream. Inother words, the amount of non-CO₂ gases removed from the multicomponentgaseous stream following selective CO₂ extraction according to thesubject invention are extremely low. For example, where themulticomponent gaseous stream is a shifted synthesis gas stream, theamount of combustible gases (i.e. H₂, CH₄ and CO) recovered is above99%, usually above 99.2% and more usually above 99.5%, where the amountrecovered ranges in many embodiments from about 99.6 to 99.8%.

Where it is desired to sequester the CO₂ clathrates produced by thesubject method, the resultant CO₂ clathrate slurry may be disposed ofdirectly as is known in the art, e.g. through placement in gas wells,the deep ocean or freshwater aquifers, (See FIG. 2) and the like, orsubsequently processed to separate the clathrates from the remainingnucleated water, where the isolated clathrates may then be disposed ofaccording to methods known in the art and the remaining nucleated waterrecycled for further use as a selective CO₂ absorbent in the subjectmethods, and the like.

Where desired, CO₂ can easily be regenerated from the clathrates, e.g.where CO₂ is to be a product, using known methods. The resultant CO₂ gasmay be disposed of by transport to the deep ocean or ground aquifers, orused in a variety of processes, e.g. enhanced oil recovery, coal bedmethane recovery, or further processed to form metal carbonates, e.g.MgCO₃, for fixation and sequestration.

In a preferred embodiment, the CO₂ hydrate slurry is treated in a mannersufficient to decompose the hydrate slurry into CO₂ gas and CO₂nucleated water, i.e. it is subjected to a decomposition step.Typically, the CO₂ hydrate slurry is thermally treated, e.g. flashed,where by thermally treated is meant that temperature of the CO₂ hydrateslurry is raised in sufficient magnitude to decompose the hydrates andproduce CO₂ gas. Typically, the temperature of the CO₂ hydrate slurry israised to a temperature of between about 40 to 50° F., at a pressureranging from about 20 to 50 atm, usually from about 40 to 60 atm. Oneconvenient means of thermally treating the CO₂ hydrate slurry is in acounterflow heat exchanger, where the heat exchanger comprises a heatingmedium in a containment means that provides for optimal surface areacontact with the clathrate slurry. Any convenient heating medium may beemployed, where specific heating media of interest include: ammonia,HCFC's and the like, with ammonia vapor at a temperature ranging from 20to 40° C. being of particular interest. Preferably, the ammonia vapor isthat vapor produced in cooling the nucleation and/or hydrate formationreactors, as described in greater detail in terms of the figures.

In a preferred embodiment, the decomposition of the CO₂ hydrates isinitiated using steam. In certain embodiments, moderate to high pressuresteam is injected directly into the CO₂ clathrate slurry. By moderatepressure steam is meant steam at a pressure ranging from about 3 to 20atm, usually from about 6 to 10 atm. Where moderate pressure steam isemployed, the temperature ranges from about 135 to 210° C., usually fromabout 160 to 182° C. By high pressure steam is meant steam at a pressureranging from about 20 to 100 atm, usually from about 40 to 60 atm. Wherehigh pressure steam is employed, the temperature of the steam rangesfrom about 215 to 400° C., usually from about 260 to 310° C. In analternative embodiment, low to moderate pressure steam is employed inthe heat exchanger without direct contact between the steam and thehydrate slurry. By low to moderate pressure steam is meant steam at apressure ranging from about 2 to 20 atm, usually from about 3 to 6 atm.Where low pressure steam is employed, the temperature of steam rangesfrom about 120 to 215° C., usually from about 135 to 160° C. A number ofadvantages are provided by these embodiments in which steam is employed.These advantages include: increased ammonia cooling efficiency,decreased use of parasitic power, the opportunity to reduce or eveneliminate water makeup requirements in those embodiments where steam isinjected directly into the hydrate slurry, and the like.

Many embodiments of the subject invention provide for extremelyefficient regeneration of moderate to high pressure CO₂ gas from the CO₂slurry in the flash reactor. Thus, the subject methods can be used toregenerate a CO₂ gaseous product that is at a pressure only slightlyless than the total system pressure. As such, the CO₂ gaseous productthat is produced from the flash reactor may range in pressure from about7 to 60 atm, usually from about 10 to 40 atm in those systems where theoverall pressure ranges from about 10 to 65 atm, usually from about 15to 45 atm. Furthermore, a substantial amount of the energy required togenerate the moderate to high pressure CO₂ product gas may be derivedsolely from the coolant medium (e.g. ammonia) that is employed elsewherein the system, e.g. in cooling of the nucleation and/or hydrateformation reactors, etc., where by substantial amount is meant at leastabout 60%, usually at least about 70% and more usually at least about80%, where in some cases as much as 85%, 90% or more of the energyrequired to produce the CO₂ product gas in the flash step may be derivedfrom the coolant medium. Thus, the subject invention efficientlyprovides for a very small pressure differential between the total systempressure and the pressure of the regenerated CO₂ gaseous product andutilizes heat generated in forming the hydrates and other heat sourcesfor regenerating the CO₂ gaseous stream. This feature provides for anextremely energy efficient process in that the ammonia or other coolantmedium is condensed at 10 to 15° C. as opposed to 25 to 40° C. which isrequired in other processes.

In certain preferred embodiments, a hydrate slurry high pressure pump isemployed to pressurize the hydrate slurry to high pressures before it issubjected to the above hydrate decomposition procedure. In theseembodiments, the hydrate slurry is pressurized to pressures ranging fromabout 75 to 170 atm, usually from about 80 to 100 atm. Any convenienthigh pressure slurry pump may be employed which is capable ofpressurizing the slurry to the requisite pressure. Benefits which areprovided by this embodiment include the ability to decompose the CO₂hydrate slurry into CO₂ above its critical pressure and water. In theseembodiments, the CO₂ liquid/gas is above its critical pressure and doesnot require further compression for disposal or utilization. Asupercritical CO₂ /water separator is also employed, e.g. a horizontalseparator.

Following separation of the CO₂ from the clathrate slurry, the resultantwater may be recycled as CO₂ partially nucleated water to separate CO₂from additional quantities of the multicomponent gaseous stream. Wherethe water is recycled for further use as nucleated water, it may benecessary to add additional quantities of water, e.g. make up water. Theamount of water that is added typically does not exceed about 0.1%, andusually does not exceed about 0.05%. In addition, CO₂ may be added.Where CO₂ is added, the amount of CO₂ that is added typically does notexceed about 6.0%, and usually does not exceed about 5.0%. In preferredembodiments, the CO₂ that is added is high pressure, substantially pureCO₂, as described above.

In the particular embodiment where the partially nucleated water isrecycled, this water may contain a homogenous dispersed or dissolvedcatalyst, such as a metal oxide, to promote hydrate precursor formation.Since nearly all of the hydrate water is recycled, there is minimal lossof the homogenous dispersed or dissolved catalyst, again making theprocess very efficient.

A variety of multicomponent gaseous streams are amenable to treatmentaccording to the subject methods. Multicomponent gaseous streams thatmay be treated according to the subject invention will comprise at leasttwo different gaseous components and may comprise five or more differentgaseous components, where at least one of the gaseous components will beCO₂, where the other component or components may be one or more of N₂,O₂, H₂ O, CH₄, H₂, CO and the like, as well as one or more trace gases,e.g. H₂ S. The total pressure of the gas will generally be at leastabout 20 atm, usually at least about 40 atm and more usually at leastabout 70 atm. The mole fraction of CO₂ in the multicomponent gaseousstreams amenable to treatment according to the subject invention willtypically range from about 25 to 65, usually from about 30 to 60, moreusually from about 0.40 to 0.50. Generally, the partial pressure of CO₂in the multicomponent gaseous stream will be at least about 5 to 6 atm,where in many embodiments the partial pressure of the CO₂ will be leastabout 10 atm and as great as 50 atm. As mentioned above, by controllingthe clathrate formation conditions of contact appropriately, contactbetween the CO₂ nucleated water and the gas can be controlled to providefor the selective formation of CO₂ clathrates, e.g. through use ofhighly nucleated water containing hydrate precursors, and perhapsdissolved or dispersed catalysts, which further aids the selectiveabsorption or adsorption of the CO₂ gas. The particular conditions whichprovide for the best selectivity with a particular gas can readily bedetermined empirically by those of skill in the art. Particularmulticomponent gaseous streams of interest that may be treated accordingto the subject invention include: oxygen containing combustion powerplant flue gas, turbo charged boiler product gas, coal gasificationproduct gas, shifted coal gasification product gas, anaerobic digesterproduct gas, wellhead natural gas stream, reformed natural gas hydrates,and the like. Where the gaseous stream is at atmospheric pressure, itwill generally be compressed to at least a minimal pressure required forCO₂ hydrate formation, e.g. about 6 atm.

Generally, the partial pressure of each of the components of themulticomponent gaseous medium will be such that CO₂ is selectivelyabsorbed or adsorbed by the nucleated water and other components arenot. As such, the partial pressure of CO₂ in the multicomponent gaseousstream will be sufficiently high and the partial pressure of each of theother components of the multicomponent gaseous stream will besufficiently low to provide for the desired selective CO₂ absorption oradsorption.

Multicomponent gaseous mediums in which the partial pressures of each ofthe components are suitable for selective CO₂ hydrate formationaccording to the subject invention may be treated directly without anypretreatment or processing. For those multicomponent gaseous mediumsthat are not readily suitable for treatment by the subject invention,e.g. in which the partial pressure of CO₂ is too low and/or the partialpressure of the other components are too high, may be subjected to apretreatment or preprocessing step in order to modulate thecharacteristics of the gaseous medium so that is suitable for treatmentby the subject method. Illustrative pretreatment or preprocessing stepsinclude: temperature modulation, e.g. heating or cooling, decompression,compression, incorporation of additional components, e.g. CO₂, and thelike.

One particular multicomponent gas of interest that may be treatedaccording to the subject invention is natural well head gas, e.g.natural gas comprising one or more lower alkyls, such as methane,ethane, butane and the like. Where the gas conditions are appropriate,CO₂ may be separated from the gas according to the subject inventiondirectly at the well head site without modification or processing of thegas. For example, if the wellhead gas has a total pressure ofapproximately 60 to 70 atm, a temperature of 0 to 5° C. and consistssubstantially of CO₂ and methane, where the amount of CO₂ present in thegas is greater than about 50 volume %, the wellhead gas can be treatedwithout modification according to the subject invention. Conversely,where the temperature of the well head gas is closer to 10° C., as longas the partial pressure of CO₂ in the methane/CO₂ mixture is at least 30atm and the partial pressure of methane is below 60 atm, the wellheadgas can be processed without pretreatment. Where the well head gasconditions are not directly suitable for treatment, the wellhead gas maybe processed to make it suitable for treatment as described above, whereprocessing includes temperature modulation, e.g. heating or cooling,decompression and/or recompression, incorporation of additionalcomponents, e.g. CO₂, and the like. For example, where the concentrationof CO₂ in the wellhead gas is from about 15 to 50% and the temperatureis from 0 to 5° C., the gas stream can be treated by decompressing it ina manner sufficient to maintain the partial pressure of the CO₂component above 10 atm and achieve a partial pressure of methane that isbelow 20 atm.

For the treatment of a shifted coal gasification product gas, theuntreated gas will typically comprise H₂, H₂ O, CO₂ and trace gases,where the mole fraction of CO₂ will range from about 0.30 to 0.45, andwill be at a temperature ranging from about 20 to 30° C. and a pressureranging from about 20 to 40 atm. The product gas will first be cooled toa temperature ranging from -30 to 0° C. and then contacted with CO₂nucleated water, as described above, to produce CO₂ depleted shiftedcoal gasification product gas and a slurry of CO₂ clathrates. Theresultant CO₂ depleted product gas and CO₂ clathrate slurry may then beseparated and the product gas used as a fuel or for chemical synthesis.Where the shifted coal gas comprises trace amounts of H₂ S, H₂ S willgenerally be present in amounts ranging from 0 to 2.0 mole percent or 0to 3.6 weight percent. In such cases, the H₂ S will either be dissolvedin the excess slurry water or will form hydrates along with the CO₂ andtherefore be separated from the multicomponent gaseous stream, obviatingthe need for subsequent H₂ S removal steps.

Yet another type of multicomponent gas that is particularly suited fortreatment according to the subject invention is powerplant gas producedby the use of pure oxygen instead of air in the combustion of organicfuels, such as coal, oil or natural gas. In a preferred embodiment, thispowerplant flue gas is compressed to 6 to 20 atm and contacted with CO₂nucleated water. The resultant hydrate slurry is nearly pure and is thenpumped to 75 to 170 atm, which can then be flashed as described above toproduce CO₂ gas and nucleated water, where the CO₂ gas can then bedisposed of or utilized, e.g. in coal bed methane recovery or enhancedoil recovery.

The subject methods and systems provide for a number of advantages.First, the subject methods provide for extremely high CO₂ removal ratesfrom the multicomponent gaseous stream. In many embodiments, the CO₂removal rate exceeds about 75%. In yet other embodiments, the CO₂removal rate may exceed about 90% or even 95%, and may be substantially100% in many embodiments. The subject invention also allows for highrecovery rates of selective non-CO₂ gases, e.g. H₂ S, in themulticomponent gaseous stream, where recovery rates of non-CO₂ gases maybe as high as 95% or higher, and in many embodiments are higher than99%. Finally, the subject invention provides for low parasitic energyuse, where the percentage of the total energy output of a given systemthat must be devoted to selective CO₂ removal may be as low as 5% orlower, and in many embodiments may be as low as 3.5% or lower.

It is evident from the above that a simple and efficient method for theselective removal of CO₂ from a multicomponent gaseous stream isprovided. By using CO₂ nucleated water containing hydrate precursors asa selective CO₂ absorbents or adsorbents, great efficiencies areachieved through reductions in the overall energy input requirements andthe number of steps necessary for complete CO₂ removal, fixation anddisposal. In particular, by using CO₂ nucleated water as the absorbentor adsorbent, CO₂ is readily removed from the gaseous stream andimmediately fixed in a form suitable for disposal. By treating theresultant CO₂ hydrate slurry to produce CO₂ gas and nucleated water,even further reductions in parasitic energy loss are obtained, wheresuch reductions stem from the use of recycled nucleated water, energyefficient recycled coolant medium, e.g. ammonia, and the like. Evenfurther benefits are obtained in preferred embodiments where at leastone of the following is present: (a) preparation of nucleated water withhigh pressure CO₂, i.e. the use of overpressure CO₂ ; (b) utilization ofa dispersed or dissolved catalyst in nucleated water to promote hydrateprecursor formation; (c) use of a hydrate reactor in which themulticomponent gaseous stream and the nucleated water are injectedsubstantially normal to each other and mix in the reaction zone, e.g.the multicomponent gaseous stream is injected along the centerline ofthe reactor and the nucleated water is injected normally to thecenterline, e.g. by injecting the water from an annulus surrounding themulticomponent gaseous stream; and (d) use of a heat decomposition stepin which steam is employed, e.g. where a low to moderate pressure steamis employed in addition to the energy extracted from the coolant medium,e.g. ammonia.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be readily apparent to those of ordinary skill inthe art in light of the teachings of this invention that certain changesand modifications may be made thereto without departing from the spiritor scope of the appended claims.

What is claimed is:
 1. A method for selectively removing CO₂ from a multicomponent gaseous stream to produce a CO₂ depleted gaseous stream, said method comprising:(1) contacting said multicomponent gaseous stream with CO₂ nucleated water containing hydrate precursors formed by an over-pressurizing process under conditions of selective CO₂ clathrate formation, whereby CO₂ is absorbed from said gaseous stream by said CO₂ nucleated water and concomitantly fixed as CO₂ clathrates upon said contacting, whereby a CO₂ depleted gaseous stream and a CO₂ clathrate slurry are produced; and (2) separating said CO₂ depleted gaseous stream from said CO₂ clathrate slurry; wherein said method is further characterized by at least one of the following:(a) preparing said nucleated water by contacting water with high pressure CO₂ gas; (b) contacting said gaseous stream with said nucleated water in a hydrate formation reactor by injecting said CO₂ nucleated water and said gaseous stream normal to each other in said reactor; and (c) employing a homogenous catalyst in said nucleated water.
 2. The method according to claim 1, wherein said method further comprises decomposing said CO₂ clathrate slurry to produce CO₂ gas and CO₂ nucleated water.
 3. The method according to claim 1, wherein said CO₂ clathrate decomposition step comprises heating said CO₂ clathrate slurry.
 4. The method according to claim 3, wherein said heating partially employs at least one of steam and energy obtained from other steps of the process.
 5. The method according to claim 3, wherein said method further comprises raising or maintaining the pressure of said CO₂ clathrate slurry to or at a high pressure prior to said heating step.
 6. The method according to claim 1, wherein said nucleated water is prepared from seawater.
 7. The method according to claim 1, wherein said nucleated water is prepared from fresh water.
 8. A method for selectively removing CO₂ from a multicomponent gaseous stream to produce a CO₂ depleted gaseous stream, said method comprising:(1) preparing CO₂ nucleated water by contacting CO₂ gas with water in a nucleation reactor; (2) contacting said multicomponent gaseous stream with said CO₂ nucleated water containing hydrate precursors formed by an over-pressurizing process in a CO₂ hydrate formation reactor under conditions of selective CO₂ clathrate formation, whereby CO₂ is removed from said gaseous stream by said CO₂ nucleated water and concomitantly fixed as CO₂ clathrates upon said contacting, whereby a CO₂ depleted gaseous stream and a CO₂ clathrate slurry are produced; (3) separating said CO₂ depleted gaseous stream from said CO₂ clathrate slurry; and (4) heating said CO₂ clathrate slurry in a manner sufficient to produce CO₂ gas and CO₂ nucleated water; wherein said method is further characterized by at least one of the following:(a) preparing said nucleated water by contacting water with high pressure CO₂ gas; (b) contacting said gaseous stream with said nucleated water in said hydrate formation reactor by injecting said CO₂ nucleated water and said gaseous stream normal to each other in said reactor; (c) using steam in said heating step; (d) maintaining or raising the pressure of said CO₂ clathrate slurry to or at a high pressure prior to said heating step; and (e) employing a homogenous catalyst in said nucleated water.
 9. The method according to claim 8, wherein said nucleated water is fresh water.
 10. The method according to claim 8, wherein said preparing said nucleated water includes contacting water with high pressure CO₂ gas.
 11. The method according to claim 8, wherein said method comprises contacting said gaseous stream with said nucleated water in said hydrate formation reactor by injecting said CO₂ nucleated water and said gaseous stream normal to each other in said reactor.
 12. The method according to claim 8, wherein said heating partially employs at least one of steam and energy obtained from other steps of the process.
 13. The method according to claim 8, wherein said method comprises raising the pressure of said CO₂ clathrate slurry to a high pressure prior to said heating step.
 14. The method according to claim 8, wherein said CO₂ nucleated water production step comprises combining CO₂ nucleated water produced by heating said CO₂ clathrate slurry with additional make up water and CO₂ gas. 